UE Mass Entity

Target engine: UE 5.8. APIs below are verified against the 5.8 headers; older forms are listed under "Deprecated — do not use".

Mass is an archetype Entity Component System for simulating thousands of agents with cache-friendly data layouts and multi-threaded processors. The core lives in the always-available runtime modules MassEntity and MassCore (plus MassSignals, MassEngine); gameplay-facing pieces (spawning, representation, movement, LOD, replication, actor bridging) come from the MassGameplay (Experimental in 5.8) plugin, and navigation/behavior from MassAI (Experimental in 5.8) and MassCrowd (Experimental in 5.8).

Context

Read .agents/ue-project-context.md if it exists (module names, conventions, enabled plugins, GAS/networking setup). Do not stop if it is missing.

Identify the area from the request and the codebase. Ask only when two plausible readings would produce different code.

Longer templates live in mass-entity-patterns.md; built-in types and fields in mass-fragment-reference.md.

Modules and Build.cs

There is no MassEntity plugin to enable. MassEntity and MassCore are runtime modules that ship with the engine — add them to Build.cs and they link. Only the gameplay layers are plugins, and those must be enabled in the .uproject.

PublicDependencyModuleNames.AddRange(new string[] {
    "Core", "CoreUObject", "Engine",
    "MassEntity",          // manager, processors, queries, commands, relations
    "MassCore",            // FMassFragment/FMassTag bases, FTransformFragment, traits
    "MassSignals",         // UMassSignalSubsystem, UMassSignalProcessorBase
    "MassCommon",          // UE::Mass::ProcessorGroupNames      (MassGameplay)
    "MassSpawner",         // UMassEntityTraitBase, AMassSpawner  (MassGameplay)
    "MassMovement",        // FMassVelocityFragment               (MassGameplay)
    "MassRepresentation",  // ISM visualization + LOD             (MassGameplay)
    "MassActors",          // UMassAgentComponent, actor bridging (MassGameplay)
    "MassNavigation",      // FMassMoveTargetFragment             (MassAI)
});

Include paths that moved into MassCore are namespaced under Mass/: Mass/EntityElementTypes.h, Mass/EntityFragments.h, Mass/EntityHandle.h, Mass/ExternalSubsystemTraits.h, Mass/ArchetypeGroup.h. MassEntityTypes.h still pulls the element bases in transitively; prefer the explicit Mass/ path.

Fragments, Tags and Archetypes

All of these derive from FMassElement and are declared in Mass/EntityElementTypes.h.

// MyMassTypes.h
#pragma once
#include "Mass/EntityElementTypes.h"
#include "MyMassTypes.generated.h"

USTRUCT()
struct FMyHealthFragment : public FMassFragment
{
    GENERATED_BODY()

    UPROPERTY(EditAnywhere, Category = "Mass")
    float Current = 100.f;

    UPROPERTY(EditAnywhere, Category = "Mass")
    float Max = 100.f;
};

USTRUCT()
struct FMyAliveTag : public FMassTag { GENERATED_BODY() };

USTRUCT()
struct FMyDeadTag : public FMassTag { GENERATED_BODY() };

USTRUCT()
struct FMySquadSharedFragment : public FMassSharedFragment
{
    GENERATED_BODY()

    UPROPERTY(EditAnywhere, Category = "Mass")
    int32 SquadID = 0;
};

Entities with the same composition share an archetype, and each fragment type is stored contiguously per chunk. Adding or removing a fragment or tag moves the entity to another archetype — that is why structural change is deferred during iteration. Sparse fragments and sparse tags exist precisely to avoid that move.

FMassEntityManager

FMassEntityManager is not a UObject; it is a struct deriving from TSharedFromThis<FMassEntityManager> and FGCObject. Reach it through UMassEntitySubsystem (a UMassSubsystemBase, itself a UWorldSubsystem) or through UE::Mass::Utils.

#include "MassEntitySubsystem.h"
#include "MassEntityManager.h"

UMassEntitySubsystem* MassSubsystem = GetWorld()->GetSubsystem<UMassEntitySubsystem>();
FMassEntityManager& EntityManager = MassSubsystem->GetMutableEntityManager();
// const access: MassSubsystem->GetEntityManager()
// free function: UE::Mass::Utils::GetEntityManagerChecked(*GetWorld())

FMassEntityHandle Entity = EntityManager.CreateEntity(ArchetypeHandle);

FMassEntityHandle Reserved = EntityManager.ReserveEntity();
EntityManager.BuildEntity(Reserved, ArchetypeHandle);

// Batch creation: hold the returned context until observers should fire.
TArray<FMassEntityHandle> Entities;
TSharedRef<FMassEntityManager::FEntityCreationContext> CreationContext =
    EntityManager.BatchCreateEntities(ArchetypeHandle, 5000, Entities);

EntityManager.DestroyEntity(Entity);
EntityManager.BatchDestroyEntities(Entities);

FMassEntityHandle::IsSet() (and its alias IsValid()) only reports that Index and SerialNumber are non-zero. To ask whether an entity really exists, use the manager:

EntityManager.IsEntityValid(Handle);   // handle refers to a live entity
EntityManager.IsEntityBuilt(Handle);   // reserved handle has been built
EntityManager.IsEntityActive(Handle);  // valid and built

Outside processor iteration, direct structural mutation is legal. The add/tag/swap calls take TNotNull<const UScriptStruct*> (RemoveFragmentFromEntity still takes a plain const UScriptStruct*, MassEntityManager.h:430), so pass T::StaticStruct() and never a possibly-null pointer.

EntityManager.AddFragmentToEntity(Handle, FMyHealthFragment::StaticStruct());
EntityManager.RemoveFragmentFromEntity(Handle, FMyHealthFragment::StaticStruct());
EntityManager.AddTagToEntity(Handle, FMyDeadTag::StaticStruct());
EntityManager.RemoveTagFromEntity(Handle, FMyDeadTag::StaticStruct());
EntityManager.SwapTagsForEntity(Handle, FMyAliveTag::StaticStruct(), FMyDeadTag::StaticStruct());

Entity storage is concurrent: FMassEntityManager::Initialize(const FMassEntityManagerStorageInitParams&) takes FMassEntityManager_InitParams_Concurrent (MaxEntityCount, MaxEntitiesPerPage) and builds a FConcurrentEntityStorage. The subsystem does this for you; only standalone managers need the call.

UMassProcessor

Subclass UMassProcessor, construct the query with *this so it registers itself, and override the two virtuals exactly as declared.

// MyMovementProcessor.h
#pragma once
#include "MassProcessor.h"
#include "MassEntityQuery.h"
#include "MyMovementProcessor.generated.h"

UCLASS()
class MYGAME_API UMyMovementProcessor : public UMassProcessor
{
    GENERATED_BODY()
public:
    UMyMovementProcessor();
protected:
    virtual void ConfigureQueries(const TSharedRef<FMassEntityManager>& EntityManager) override;
    virtual void Execute(FMassEntityManager& EntityManager, FMassExecutionContext& Context) override;
private:
    FMassEntityQuery MovementQuery;
};

// MyMovementProcessor.cpp
#include "MyMovementProcessor.h"
#include "MassCommonTypes.h"        // UE::Mass::ProcessorGroupNames
#include "MassExecutionContext.h"
#include "MassMovementFragments.h"   // FMassVelocityFragment, FMassMovementParameters
#include "MassSignalSubsystem.h"
#include "Mass/EntityFragments.h"    // FTransformFragment
#include "MyMassTypes.h"             // FMyHealthFragment, FMyDeadTag, FMySquadSharedFragment

UMyMovementProcessor::UMyMovementProcessor()
    : MovementQuery(*this)
{
    ProcessingPhase = EMassProcessingPhase::PrePhysics;
    ExecutionFlags = static_cast<int32>(EProcessorExecutionFlags::AllNetModes);
    ExecutionOrder.ExecuteInGroup = UE::Mass::ProcessorGroupNames::Movement;
    ExecutionOrder.ExecuteAfter.Add(TEXT("MassApplyMovementProcessor")); // class FName: no U prefix
    bRequiresGameThreadExecution = false;
}

EMassProcessingPhase: PrePhysics, StartPhysics, DuringPhysics, EndPhysics, PostPhysics, FrameEnd, MAX.

EProcessorExecutionFlags: None, Standalone, Server, Client, Editor, EditorWorld, AllNetModes (Standalone|Server|Client), AllWorldModes (AllNetModes|EditorWorld), All.

FMassProcessorExecutionOrder carries ExecuteInGroup (an FName) plus ExecuteBefore and ExecuteAfter arrays of processor or group names. A processor's name is its class FName without the U prefix (MassProcessorDependencySolver.cpp:560) — prefer UOther::StaticClass()->GetFName(); a mistyped name just becomes a dummy node with a Log-verbosity message (:705), so the ordering is silently lost. Named groups come from UE::Mass::ProcessorGroupNames: UpdateWorldFromMass, SyncWorldToMass, Behavior, Tasks, Avoidance, ApplyForces, Movement.

FMassEntityQuery

Declare requirements in ConfigureQueries — never in the constructor, because the query is bound to the entity manager just before ConfigureQueries runs. Queries built with FMassEntityQuery(UMassProcessor& Owner) are already registered; a default-constructed member still needs RegisterQuery(Query).

void UMyMovementProcessor::ConfigureQueries(const TSharedRef<FMassEntityManager>& EntityManager)
{
    MovementQuery.AddRequirement<FTransformFragment>(
        EMassFragmentAccess::ReadWrite, EMassFragmentPresence::All);
    MovementQuery.AddRequirement<FMassVelocityFragment>(
        EMassFragmentAccess::ReadOnly, EMassFragmentPresence::All);
    MovementQuery.AddRequirement<FMyHealthFragment>(
        EMassFragmentAccess::ReadOnly, EMassFragmentPresence::Optional);
    MovementQuery.AddTagRequirement<FMyDeadTag>(EMassFragmentPresence::None);
    MovementQuery.AddSharedRequirement<FMySquadSharedFragment>(
        EMassFragmentAccess::ReadOnly, EMassFragmentPresence::All);
    MovementQuery.AddConstSharedRequirement<FMassMovementParameters>(
        EMassFragmentPresence::All);
    MovementQuery.AddSubsystemRequirement<UMassSignalSubsystem>(
        EMassFragmentAccess::ReadWrite);
}

AddChunkRequirement<T>(Access, Presence) binds a FMassChunkFragment. SetChunkFilter(const FMassChunkConditionFunction&) takes a TFunction<bool(const FMassExecutionContext&)> evaluated once per chunk, so the predicate may read chunk fragments, shared fragments and constant data — never per-entity fragments.

Iteration and FMassExecutionContext

void UMyMovementProcessor::Execute(FMassEntityManager& EntityManager, FMassExecutionContext& Context)
{
    MovementQuery.ForEachEntityChunk(Context, [](FMassExecutionContext& Context)
    {
        const int32 NumEntities = Context.GetNumEntities();
        const TArrayView<FTransformFragment> Transforms = Context.GetMutableFragmentView<FTransformFragment>();
        const TConstArrayView<FMassVelocityFragment> Velocities = Context.GetFragmentView<FMassVelocityFragment>();
        const float DeltaTime = Context.GetDeltaTimeSeconds();

        for (int32 Index = 0; Index < NumEntities; ++Index)
        {
            Transforms[Index].GetMutableTransform().AddToTranslation(Velocities[Index].Value * DeltaTime);
        }
    });
}

The lambda takes only FMassExecutionContext&; there is no entity-manager parameter. Other accessors: GetEntities() (TConstArrayView<FMassEntityHandle>), GetFragmentView<T>(), GetMutableFragmentView<T>(), GetChunkFragment<T>(), GetConstSharedFragment<T>(), GetMutableSharedFragment<T>(), GetSubsystem<T>(), GetMutableSubsystem<T>() and Defer().

Parallel: ParallelForEachEntityChunk(Context, Lambda, FMassEntityQuery::EParallelExecutionFlags::Default); the nested enum's flags are Default, Force and AutoBalance (MassEntityQuery.h:67).

Time-slicing: ForEachEntityChunk(Context, Limiter, Lambda) takes a UE::Mass::FExecutionLimiter& built from an entity count, stops once that many entities have been visited (always finishing the current chunk) and resumes from the stored position on the next call. Keep the limiter alive across frames as a processor member — a fresh one restarts from the beginning, and the limiter does not notice entities created or destroyed between calls.

Deferred Commands

Inside ForEachEntityChunk never mutate composition through the entity manager: the entity moves archetype and the views you are holding dangle. Queue the change on Context.Defer().

MovementQuery.ForEachEntityChunk(Context, [](FMassExecutionContext& Context)
{
    const TConstArrayView<FMassEntityHandle> Entities = Context.GetEntities();
    const TConstArrayView<FMyHealthFragment> Healths = Context.GetFragmentView<FMyHealthFragment>();

    for (int32 Index = 0; Index < Context.GetNumEntities(); ++Index)
    {
        if (Healths[Index].Current <= 0.f)
        {
            Context.Defer().SwapTags<FMyAliveTag, FMyDeadTag>(Entities[Index]);
            Context.Defer().RemoveFragment<FMyHealthFragment>(Entities[Index]);
        }
    }
});

Convenience calls on FMassCommandBuffer: AddFragment<T>, RemoveFragment<T>, AddTag<T>, RemoveTag<T>, SwapTags<TOld, TNew>, AddElements<T...>, RemoveElements<T...>, DestroyEntity, DestroyEntities. The add/remove helpers forward to FMassCommandAddElements<T...> / FMassCommandRemoveElements<T...>, which handle any mix of fragments and tags in a single entity move (SwapTags uses FMassCommandSwapTagsInternal, MassCommandBuffer.h:264).

To add a fragment and its value, push the command type directly:

Context.Defer().PushCommand<FMassCommandAddFragmentInstances<FMyHealthFragment>>(
    Entities[Index], FMyHealthFragment{});

For arbitrary deferred work push a lambda through FMassDeferredCommand<OpType>: Context.Defer().PushCommand<FMassDeferredSetCommand>([Entity](FMassEntityManager& Manager) { ... }); (aliases FMassDeferredCreate/Add/Remove/ChangeComposition/Set/DestroyCommand, MassCommands.h:1686-1744). For a reusable typed command, subclass FMassBatchedCommand, override Run(FMassEntityManager&) (the const Execute is UE_DEPRECATED(5.7), MassCommands.h:202) and call PushUniqueCommand(TUniquePtr<FMassBatchedCommand>&&).

Commands flush by EMassCommandOperationType bucket, not enum order: Create → Add → ChangeComposition → Set → Remove and Destroy (one shared bucket, push order) → the unclassified None last (MassCommandBuffer.cpp:110-119). That ordering guarantees entities exist before they are modified and fragments exist before values are written.

UMassObserverProcessor

Observers run when an observed fragment or tag is added or removed. Fill ObservedTypes (an array — one observer can watch several types) and ObservedOperations.

// MyHealthObserver.cpp
UMyHealthObserver::UMyHealthObserver()
    : ObserverQuery(*this)
{
    ObservedTypes.Add(FMyHealthFragment::StaticStruct());
    ObservedOperations = EMassObservedOperationFlags::Add;
}

EMassObservedOperationFlags: None, AddElement, RemoveElement, CreateEntity, DestroyEntity, plus the composites Add (= AddElement | CreateEntity), Remove (= RemoveElement | DestroyEntity) and All. Use Add/Remove unless you deliberately want to ignore entity creation — AddElement alone does not fire for entities that are born with the fragment.

Override Register() to customise which types get registered, and read them back with GetObservedTypes() or GetSingleObservedTypeChecked(). Full observer template in mass-entity-patterns.md.

Single-Entity Access and FEntityBuilder

FMassEntityView gives typed access to one entity. It is transient — archetype memory relocates, so build a fresh view every time and never cache one.

if (EntityManager.IsEntityValid(Handle))
{
    FMassEntityView View(EntityManager, Handle);
    if (FMyHealthFragment* Health = View.GetFragmentDataPtr<FMyHealthFragment>())
    {
        Health->Current -= Damage;
    }
    const bool bDead = View.HasTag<FMyDeadTag>();
}

UE::Mass::FEntityBuilder (MassEntityBuilder.h) composes an entity fluently and commits once, which beats repeated AddFragmentToEntity calls.

#include "MassEntityBuilder.h"

UE::Mass::FEntityBuilder Builder(EntityManager);
Builder.Add<FTransformFragment>(SpawnTransform);
Builder.Add<FMyHealthFragment>();
Builder.Add<FMyAliveTag>();
FMyHealthFragment& Health = Builder.GetOrCreate<FMyHealthFragment>();
Health.Current = 50.f;
const FMassEntityHandle NewEntity = Builder.Commit();

SetForceDeferredCommit(true) routes the commit through the command buffer; CommitAndReprepare() reuses the builder for the next entity.

Relations and Archetype Groups

Relations model typed links between entities. A relation type derives from FMassRelation (itself an FMassTag); the engine ships FMassChildOfRelation with FMassChildOfFragment in Relations/MassChildOf.h. Instances are created and queried through UE::Mass::FRelationManager, reached with EntityManager.GetRelationManager().

#include "MassRelationManager.h"
#include "Relations/MassChildOf.h"

UE::Mass::FRelationManager& Relations = EntityManager.GetRelationManager();
Relations.CreateRelationInstance<FMassChildOfRelation>(ChildEntity, ParentEntity);
TArray<FMassEntityHandle> Children = Relations.GetRelationSubjects(
    FMassChildOfRelation::StaticStruct(), ParentEntity);

Deferred equivalent: Context.Defer().PushCommand<FMassCommandMakeRelation<FMassChildOfRelation>>(ChildEntity, ParentEntity). Builders take AddRelation<T>(OtherEntity).

Archetype groups let a query process archetypes in a controlled order. EntityManager.FindOrAddArchetypeGroupType(GroupName) returns a UE::Mass::FArchetypeGroupType; EntityManager.BatchGroupEntities(UE::Mass::FArchetypeGroupHandle(GroupType, UE::Mass::FArchetypeGroupID(N)), Entities) assigns entities (Mass/ArchetypeGroup.h:100); Query.GroupBy(GroupType) (optionally with a sort predicate) makes iteration walk groups together.

Traits, Config Assets and Spawning

UMassEntityConfigAsset (a UDataAsset) holds a list of traits that build an entity template. Traits subclass UMassEntityTraitBase and implement BuildTemplate; both virtuals are const and ValidateTemplate takes three parameters.

// MyHealthTrait.h
#pragma once
#include "MassEntityTraitBase.h"
#include "MyHealthTrait.generated.h"

UCLASS(meta = (DisplayName = "My Health"))
class MYGAME_API UMyHealthTrait : public UMassEntityTraitBase
{
    GENERATED_BODY()
public:
    UPROPERTY(EditAnywhere, Category = "Health")
    float DefaultHealth = 100.f;

protected:
    virtual void BuildTemplate(FMassEntityTemplateBuildContext& BuildContext,
                               const UWorld& World) const override;
    virtual bool ValidateTemplate(const FMassEntityTemplateBuildContext& BuildContext,
                                  const UWorld& World,
                                  FAdditionalTraitRequirements& OutTraitRequirements) const override;
};

FMassEntityTemplateBuildContext (declared in MassEntityTemplateRegistry.h) offers AddFragment<T>(), AddFragment_GetRef<T>(), AddTag<T>(), AddChunkFragment<T>(), AddSharedFragment(const FSharedStruct&), AddConstSharedFragment(const FConstSharedStruct&) and AddTranslator<T>().

UMassAssortedFragmentsTrait (MassAssortedFragmentsTrait.h) adds an editor-authored list of fragments and tags without any C++. AMassSpawner references config assets through EntityTypes and drives Count, DoSpawning() and DoDespawning().

Representation and LOD

UMassRepresentationSubsystem pools Instanced Static Mesh components and spawned actors so thousands of entities render without one actor each.

EMassLOD::Type is High, Medium, Low, Off, Max, stored as TEnumAsByte<EMassLOD::Type>; visibility is EMassVisibility (CanBeSeen, CulledByFrustum, CulledByDistance, Max). UMassMovableVisualizationTrait and UMassStationaryVisualizationTrait configure meshes, actor classes and LOD distances.

Thread-safety for anything a query touches is declared through traits in Mass/ExternalSubsystemTraits.h. TMassExternalSubsystemTraits defaults GameThreadOnly = true; TMassSharedFragmentTraits defaults GameThreadOnly = false. TMassFragmentTraits has no threading flag — only AuthorAcceptsItsNotTriviallyCopyable = false (Mass/ExternalSubsystemTraits.h:57-66). Specialise only when the default is wrong:

template<>
struct TMassExternalSubsystemTraits<UMyMassSubsystem> final
{
    enum
    {
        GameThreadOnly = false,
        ThreadSafeWrite = false
    };
};

Crowd, Navigation and Behavior

UMassCrowdSubsystem (MassCrowd, Experimental in 5.8) tracks ZoneGraph lane occupancy, density and waiting slots. Entities carry FMassCrowdTag and FMassCrowdLaneTrackingFragment; UMassCrowdMemberTrait wires them up. The subsystem specialises TMassExternalSubsystemTraits<UMassCrowdSubsystem> with GameThreadOnly = false, so parallel processors may declare it via AddSubsystemRequirement.

Movement targets live in FMassMoveTargetFragment (MassNavigationFragments.h, module MassNavigation). ZoneGraph itself and Smart Objects are covered by ue-ai-navigation.

Entity behavior runs on State Trees: UMassStateTreeTrait attaches the asset, UMassStateTreeProcessor derives from UMassSignalProcessorBase and ticks only signalled entities, and FMassStateTreeSharedFragment is a const shared fragment. Signal an entity with UMassSignalSubsystem::SignalEntity(FName SignalName, FMassEntityHandle Entity) — signal name first. Task and evaluator authoring belongs to ue-state-trees.

Deprecated — do not use

Common Mistakes

Treating Mass as a plugin: MassEntity and MassCore are runtime modules with no .uplugin. Adding "MassEntity" to Build.cs is enough; only MassGameplay, MassAI and MassCrowd need enabling in the .uproject.

Wrong header for FTransformFragment: it lives in Mass/EntityFragments.h (module MassCore). MassCommonFragments.h merely includes it; MassCommon and MassEntity list MassCore as a public dependency so it resolves transitively, but include Mass/EntityFragments.h and list MassCore explicitly rather than relying on that.

Structural change during iteration: calling EntityManager.AddFragmentToEntity(...) inside ForEachEntityChunk relocates the entity and invalidates every view in flight. Use Context.Defer().AddFragment<T>(Entity).

Requirements declared in the constructor: the query is bound to its entity manager just before ConfigureQueries runs, so AddRequirement in the constructor trips the "Modifying requirements before initialization" check. Declare requirements only in ConfigureQueries.

Forgetting to register a default-constructed query: FMassEntityQuery MyQuery; silently matches nothing until RegisterQuery(MyQuery) runs. Prefer MyQuery(*this) in the constructor initializer list.

Caching an FMassEntityView: views point into archetype memory that moves. Rebuild the view from the handle at each use.

IsSet() as an existence test: a handle to a destroyed entity still reports IsSet()/IsValid() true. Ask EntityManager.IsEntityValid(Handle).

Access mismatch: requesting ReadOnly and then calling GetMutableFragmentView<T>() asserts. Match the view to the declared EMassFragmentAccess.

Observing only AddElement: entities created with the fragment already present raise CreateEntity, not AddElement. Use EMassObservedOperationFlags::Add.

Touching UObjects from a parallel processor: set bRequiresGameThreadExecution = true, or declare the subsystem through AddSubsystemRequirement and give it a TMassExternalSubsystemTraits specialisation that states the truth.

Related Skills

• ue-state-trees — State Tree assets, tasks, evaluators and the Mass State Tree schema
• ue-ai-navigation — ZoneGraph, Smart Objects, NavMesh and perception
• ue-async-threading — task graph, ParallelFor and thread-safety fundamentals
• ue-actor-component-architecture — actors and components on the other side of UMassAgentComponent
• ue-cpp-foundations — USTRUCT/UCLASS reflection, subsystems, module layout
• ue-animation-system — animation for entities promoted to actors or skinned instances
• ue-procedural-generation — PCG and ISM authoring that feeds Mass spawning
• ue-mover — the Mover plugin: movement modes, layered moves and rollback networking